(1) Field of the Invention
The present invention relates to an ion implanter, and particularly to an ion implanter which is suited for implanting ions with a heavy current.
(2) Description of the Prior Art
In an ion implanter, ions generated from an ion source are introduced into a magnetic field where they are dispersed according to their mass numbers. Among the ions thus dispersed, those of predetermined mass numbers are allowed to pass through slits selectively, and are implanted into a sample.
In recent years, there has been a strong demand to provide an ion implanter that is capable of implanting ions with a heavy current on the order of 10 milliamperes. To implant ions with a heavy current, it is essential that ions generated from an ion source are able to arrive at the sample with as little loss as possible, i.e., by keeping the ion transmission factor as high as possible.
To increase the ion transmission factor, it is important to utilize the convergence of ions in the dispersion direction thereof (hereinafter referred to as the radial direction) as well as to utilize their convergence in a direction at right angles thereto (hereinafter referred to as the axial direction). A mass-dispersing magnetic field also exhibits a converging function in the radial direction. To converge the ions in the axial direction, on the other hand, a specific device is usually required. That is, a mass-dispersing magnetic field must be formed in the plane of incidence of ions so that the angle of incidence of ions upon that plane of incidence has a positive value. Furthermore, the plane of emission of ions in the mass-dispersing magnetic field must be formed so that the angle of emission of ions of a predetermined mass number which are emitted from the plane of emission has a positive value. This is based upon a prerequisite that ions emitted from the ion source toward a mass-dispersing magnetic field should travel nearly parallel or diverge slightly in the axial direction.
According to studies conducted by the inventors, however, ions emitted from the ion source toward the mass-dispersing magnetic field do not necessarily travel parallel or diverge slightly in the axial direction, but converge slightly as travel, so that the ions converge only poorly on the sample in the axial direction. Furthermore, the convergence of ions varies according to the angle of emission of ions emitted from the ion source in the axial direction (opening angle).